After incubation the reaction is quenched and extracted. The mixture is then centrifuged. The top acetonitrile extract layer is then transferred to a 96-well (Nunc, polypropylene) plate. Then extract is injected onto a liquid chromatography-tandem mass spectrometry (LC-MS/MS) API 3200 system for chromatographic separation and measurement of analytes. The MS/MS is operated in the multiple reaction monitoring (MRM) negative mode to follow the precursor to product species transitions for (13)C(2)-labeled UDP-galactose (567.0 to 322.9 m/z) and internal standard of UDP-n-acetylglucosamine (606.2 to 384.7 m/z). The ratio of the extracted peak area of (13)C(2)labeled UDP-galactose to its internal standard UDP-n-acetylglucosamine as determined by liquid chromatography-tandem mass spectrometry is used to calculate the concentration of product analyte in the sample. The concentration of the product is then normalized using the calculated hemoglobin concentration to determine the patient's enzyme level in nmol/h/mg of hemoglobin.(Unpublished Mayo method)